Verify offscreen canvas rendering is not fingerprintable

Once this feature has been tested, we can re-enable the gfx.offscreencanvas.enable pref.

Original ticket:

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1744854

Offscreencanvas was deemed inoffensive because it didn't have toDataURL:

FYI: see 1422862#c5 and comment 4

edit: TL;DR: not an issue

(#18599 (comment 2815213))

Yes, this sounds like we misunderstood the feature when we initially filed this bug (so as not to forget about it.) If there is no way for an OffscreenCanvas to actually give its rendered content via an API (like there is for canvas through e.g. toDataURL), and all it can do it stick it into a canvas (where permission prompts work) or another object that can't be read via Javascript (like an <img> tag) - then we are fine here.

Now it has one!

So, more tests needed (hopefully everything is already fine).